Composition and method of inhibiting the corrosion of ferrous equipment used in the regeneration and boiling of alkali metal solutions



Sept. 4, 1956 T. A. MATTHEWS 2,751,755 COMPOSITION AND METHOD OFINHIBITING THE CORROSION OF FERROUS EQUIPMENT USED IN THE REGENERATIONAND BOILING OF ALKALI METAL SOLUTIONS Filed Oct. 28, 1952 4 Sheets-Sheet1 w'r. LOSS, Mg/Gm TIME, HOURS OOI HOUR

FLANGE mo/v FIG. I

INVENTOR.

By THOMAS A. MATTHEWS ATTORNEY p 1956 T. A. MATTHEWS n 2,

COMPOSITION AND METHOD OF INHIBITING THE CORROSION OF FERROUS EQUIPMENTUSED IN THE REGENERATION AND BOILING OF ALKALI METAL SOLUTIONS FiledOct. 28, 1952 4 Sheets-Sheet 2 WT. Loss, Mg/Cm TIME, HOURS o 5 N u a 01m 0 8 8 8 8 8 HOURS FIG. 2

- INVENTOR.

BY THOMAS A. MATTHEWS ATTORNEY p 1956 T. A. MATTHEWS n 2,761,755

COMPOSITION AND METHOD OF INHIBITING THE CORROSION OF FERROUS EQUIPMENTUSED IN THE REGENERATION AND BOILING OF ALKALI, METAL SOLUTIONS FiledOct. 28, 1952 4 Sheets-Sheet 3 WT. LOSS, Mg/Gm V8.

' 0.8 TIME, HOURS Mg/(Jm 0 5 N 01 4s 8 as o 8 8 8 o 8 HOURS CARPENTER 20FIG-3 INVENTQR.

BY THOMAS AMATTHEWS ATTORNEY I Sept. 4, 1956 T. A. MATTHEWS llCOMPOSITION AND MET 2,761 ,765 HOD OF INHIBITING THE CORROSION OFFERROUS EQUIPMENT USED IN THE REGENERATION AND BOILING OF ALKALI METALSOLUTIONS Filed Oct. 28, 1952 4 Sheets-Sheet 4 WT. Loss, Mg/Cm I vs. Q9TIME, HOURS K o 6 N x/ e O E o 5 E /3 04 0.2 I 9 /4 m o/ vv g 7 l .a I5/ 2, 0 a I 1 /2\ HOURS AVERAGE 01- Mfr/:2, ZVGO/VEL a NICKEL INVENTORBY THOMAS A. MATTHEWS ATTORNEY F ALI METAL SOLUTIONS Thomas A. MatthewsII, Crystal Lake, lik, assignorto The Pure Oil Company, Chicago, 111., acorporation. of Uhio Application October 28, 1952, Serial No. 317,277

8 Claims. c1. 23-484) This invention relates to a method and a novelinhibitor for reducing the corrosion of ferrous metal equip ment used inthe processing and handling of solutions containing active percentagesof caustic alkali. Specifically the invention comprises a method ofinhibiting the corrosion of ferrous equipment used in the regenerationand boiling of alkali solutions containing large and active amounts ofalkali, which alkali solutions may or may not contain solubilitypromoters and oxidation catalysts for the sweetening of hydrocarbon oilsor ingredients added to facilitate the removal of sulfur compounds fromhydrocarbon oils and gasoline.

Strong alkali solutions, particularly aqueous or alcoholic sodium andpotassium hydroxide solutions having per cent or more by weight ofactive alkali present are known to be corrosive to metal parts,especially ferrous metal parts of equipment used to handle the alkalisolutions. The extent of corrosive action of the alkali depends upon itsconcentration, the temperature and pressure of the system, and the rateof flow or impinging action of the solution on the metal surfacesinvolved. In the treatment of hydrocarbon oils and distillates for thepurpose of removing sulfur compounds or transforming lobnoxious sulfurcompounds into non-odorous form, it is necessary to regenerate thealkali solutions. This re generation is accomplished by heating, steamstripping, or air blowing conducted at temperatures above 60 F. and ashigh as or higher than the boiling point of the alkali solution. Duringsuch treatment, the alkali solution is generally circulated through apacked tower or through a bubble tower, and portions thereof recycled toinsure adequate regeneration. This necessitates the use of piping andpumps to convey the heated alkali solutions. Severe corrosion of thepumping equipment and metal conduits has been experiencedin suchoperations. The used alkali solutions from such operations containvarious sulfur compounds including mercaptans, mercaptides, anddisulfides which tend to increase the corrosiveness of the solutions. Inaddition, certain amounts of dissolved oxygen, various solubilitypromoters, oxidation catalysts, along with bits of extraneous matter,may be present which increase the corrosiveness and add to thedifficulties attendant to attaining a regenerated alkali which is clearand clean, free of sedimentary materials, and readily reused. Theboiling and steam stripping to which these alkali solutions aresubjected during regeneration presents an atmosphere which is especiallycorrosive to ferrous metal surfaces and results in many instances in theformation of precipitates which render it colored and often containingsediments making it'unfit for contacting hydrocarbons.

To increase the life of the pumps, conduits, and other handlingequipment subjected to the above conditions, it has been the practice tofabricate these pieces of equipmerit from corrosionresistant stainlesssteel alloys at considerable expense. Even certain stainless steelalloys are attacked by hot caustic under extreme conditions and item theuse of a noble metal alloy promotes the severe States Patent 2 corrosionof adjacent parts made of base metals. During the course ofinvestigating the rate of corrosion of ferrous metals in contact withboiling strong caustic alkali solutions, the observation was made thatthe rate of corrosion increases rapidly during the first few hours ofcontact and then levels oif at a much reduced rate with the passage "oftime. A study of this phenomenon was made by applicant in order to findan inexpensive means for overcoming the corrosion problem.

One explanation for the decreased rate of corrosion over extendedperiods of time is the formation of a protective film over the metalsurface which stops or slows up further attack by the alkali. In aneffort to find conditions which demonstrate the true rate of corrosionof these ferrous metals in !order to study the actual effective.

ness of experimental oxidation and corrosion inhibitors, experimentswere conducted using an abrasive in the form of ferric oxide in thealkali solutions under test on the theory that the abrasive would removethe partially )rotec tive metal film that seemed to be forming. It wasfound that, contary to these expectations, the iron oxide acted in anopposite manner and produced a marked reduction in the corrosion rate.The degree of reduction in corrosion ratewas further found to be amaximum with certain optimum concentrations of the iron oxide present inthe alkali solution.

Accordingly, it is a fundamental object of the present invention toprovide a method of preventing the corrosion which takes place in theboiling of active alkali solutions.

A second object of the invention is to pnovide a method of preventingcorrosion of equipment used in the regeneration of alkali sweeteningsolutions or alkali desulfurizing solutions which have been used-in thetreatment of hydrocarbons.

A further object of the invention is to provide a method of inhibitingthe corrosion occurring during the regeneration of alkali solutions andmaintaining the solutions in clear, clean, usable condition.

Another object is to provide a composition for use in mitigatingcorrosion of ferrous metal surfaces by alkali solutions.

Other objects and advantages of the invention will appear in part or beobvious from the following description.

The invention, accordingly, comprises the discovery that the corrosiveaction .of alkali solutions on ferrous metal surfaces may be mitigatedby the presence of small but sufficient amounts of red iron oxide(FezOs) and/or the so-called alkali metal ferrites, sodium, potassium,and lithium feirites, which may be designated by the formula NazFezOi.The invention has as its embodiments, the process for inhibiting thecorrosion of ferrous equipment used in handling alkali solutions,including such processes as the regeneration of alkali used in treatinghydrocarbon oils; and the product therefor comprising a compositioncharacterized by its utility in mitigating this type of cor- 1'0S1011.

The process includes the several steps to be described and the relationof one or more of such steps to each of the others thereof, which willbe exemplified in the method hereinafter described and defined in theclaims.

In the drawings,

Figure 1 is a graph of weight loss of flange iron samples in contactwith hot caustic solution plotted as ordinates against the time in hoursplotted as abscissas.

Figure 2 is a graph of weight loss vs. time in hours as determined forstainless steel 304.

Figure 3 is a graph of weight loss vs. time in hours as determined forCarpenter 20, a stainless steel alloy.

Figure 4 represents a graph of the average weight losses i plotted asabscissas. f

, Typical examples jalkalisolut'ions may cause corrosion problems asatomic-1 imsflfisnd newbie may b o er o e b e: seem the presentinvention are thosejproeesses employi g causti I scrubbing: to removeIhydrogen sulfide and the, majority of the more} solu Mercapsollprocess.

potassium hydroxide'or may v with hot active alkali solutions.from'whatever source, it

j areusedthree Mercapsol process din - about 3 per cent of organiccatalyticjagents, such as; hydro qui'none, naphthoquinoneg wood tars,catechol, aromatic quinone forr ning' compounds, an,thraquinone, ndderiv hich, have. been .found useful as accelerators. 1

li solutionsm'ayalso contain; thiophenoljie compounds,

unsaturated organic aci s, reactionproducts alkali and natural orsynthetic shellac, been; acidic 'pe roleuni oxidation products, alkalimetal s'alts of high ?boil;ngtar; 1

acid oils,.and alkali metal phenol f ana mia, vaeca gum,

r a htlmbof er; aictersfsmpies in c'ontactw hfhot%c}austic 1 j solutionplotted: as ord nates against-the time hours i of Imdust rial. assess;wherein. hot

I I I lkai sweetening of drocarbons includes generally an aqueoussolvent which; may, or. may not eontain an alcohol, having 55 to 50. per'cent of: caustic alkali 1 1 usually in% the form-of sodium; hydroxideand depending 1 thiocresols, alkali metal soaps of naphthenic acids: andof will be: demonstratedfbyrefierence to alkali solutions which 1 v Thetype of solution .1 1

having a tight fitting: Tygon tubingigasket'was equipped 1 japan thetype ot treating: process; contemplated, up to "of orros'ion under:conditons of, noagitatiommild agitae 1 I I I ties; and extremeagitation.i For the: latter purpose, the 1 1 i apparatus was equipped with a PMmotor drivng f I s eering shaft through an idlerpulley, which shaftsusaj I pended a disc, type. turbin'e inimmersion contact withthei 1 1at tives :of these quinones' and; quinoneeforming? compounds f I i Thesolution may also contain other; ingredients 1 i which have the effectof improving the effieiency of the operations for the regeneration ofused alkali solutions therefrom are those described in United StatesPatents extraction. Typical of the processes employing this typeofsolution in hydrocarbontreatment and also including- 2,292,636 of August11, 1942, to Lawrence M. Henderson 1 and George W. Ayers, Jr.;.2,297,621 of September 29, 1942, to Lawrence M. Henderson and George W.Ayers, .lr.; and 2,314,919 of March 30, 1943, to Donald C. Bond, GeorgeW. Ayers, In, and Lawrence M. Henderson.

It is known in the prior art to use many different inorganic salts asinhibitors against various corrosive atmospheres in various systems.United States Patent 2.297,666 of September 29, 1942, to Aaron Wachter,employs sodium nitrite in oil pipe lines. United States Patent 2,153,952of April 11, 1939, to Alfred L. Bayes, em- 1 ploys sodium nitrate inanti-freeze solutions and Patent 2,135,160 dated November 1, 1938, toHerman Beekhuis, discloses sodium dichrornate to inhibit thecorrosiveness of ammonium nitrate systems. The use of red iron oxide asa paint pigment is well established and numerous metal protectivepigments comprising metal oxides have been used as constituents ofprimary coats applied to metallic surfaces for the purpose of inhibitingor passivating various kinds of corrosive atmospheres. Lead and zincpigments, for example, are known for their inhibitive nature.

iron oxides are generally known as neutral pigments. However, it hasbeen observed that strong caustic solutions will actually remove rediron oxide paint and many other protective coatings from metal surfaces.

The state of the art previously outlined emphasizes the fact that theproblem of the inhibition of corrosion is one of empirical phenomenonand each particular situation or environment creates its own peculiarcorrosion problem requiring specific and exacting solutions. Eachproblem of the inhibition of corrosion in a given environment,therefore, calls for a process which is designed specifically to meetthat problem. A given corrosion Z and Louis R. :Mazurk, .it is pointedout thatsodiurn.phos I 3 Worse than uselessin preventing; the corrosion.of ferrous;

. metal solutions. That patent is. directed to the use. of i alkalimetal 'nitrites to reduce; the corrosionf of. boiling. l caustic alkalisolutions on the. ferrous vessels. i The present inventionand: itsapplicab'lity to 1 solution, known as: Mercapsol solution, maintained 5at: about 230; boiling tem eraturelg for from o hours to astrnuch as.12.35 hours contact time. I I I v t i I The Mercapsol solution {used inthese tests contained I 1 f 19.3 per cent sodium hydroxide, Z 10., percent; cresols, 15;

. 41 inhibitor may be very useful in; a certain. application but wh w ulI I I I I I f United States Patent'2,5:56, 38T I oflnne 12, 1951.10GeorgeEW. Ayers; Erskine 1E; Harton,

pirate; and sodium gchromate, which} are very well known I 1 inorganicsalt corrosion} inhibitors, were found g to be tmrnent. used in theregeneration and bojilingofalkali I the; inh' I of corrosion-of ferrousmaterials by causticsolutions I ontaining various. extraneous materials;is best under- 1 tali solution; for} rotating samples therein. Variousinples of ferrous metals iand; ferrous metal alloys were;

subected to contact with a typical corrosive: caustic alkali? stood anddemonstrated by 5 reference: to. the following I I I amples and tab 1s;, I I ,I I I I I 1 1 :A .five gallon stainless; steel 1pai percentnapht-henic acids, and 55.7 per cent water. The. I l A Baum gravitywas maintained at approximately 255 by daily addition of water, whichwas lost at about 0.3 Weight per cent per day. During the runs, thetemperature remained between 220 and 230 F. Four removable bafiles wereused to prevent vortexing during mixing. Teflon plastic washers wereused to suspend the samples and insulate the bolted samples from theturbine. The Mercapsol solution had no efiect on this plastic afterweeks of immersion.

Each sample before testing was first rubbed with steel wool and emerycloth, then pickled in hot hydrochloric acid solution, rinsed with waterand acetone, dried, and weighed. After immersion for about hours, thesamples were removed, washed with water and acetone, and dried. Duringthis operation they were rubbed briskly with a cloth to remove anyreasonably loose scale. Any adherent scale remaining was included in theweight determinations made. The Weight of each sample was taken and thesamples were immersed for further contact with the alkali. Thisprocedure provided data relating weight loss with time. All samples weregiven this same treatment regardless of the degree of agitation whichwas applied during the tests.

The condition of no agitation represents one wherein the samples werehung in a static solution. The condition of mild agitation wherein thesamples are hung in a solution surrounding a mixing turbine andconvection currents are allowed to mildly agitate the solution.Conditions of more extreme agitation are represented by the amount oftravel in lineal feet per second of the samples when fastened to themixing turbine.

The following table sets forth. an analysis of the samples of ferrousmaterials used in the experiments.

F. with varying degrees of agitation, it is seen al immersion ofstainless steels 410, 316, 316

of ferrous metal alloys in contact with Mercapsol solution at 230 thatthe tot TABLE I (rolled plate) 91 5 9660202 LILLLHWLOJLO Chemicalanalysis of steels and nickel alloys ClSiMn S 29 60 5 5 5406730012QQLQQLQQQMLQQO 0 00 0 0 0 Q0 0 &

Type of Steel Flange iron.

NickelWrou M 6, where chosen Some of the metal samples selected, as, forexample, Carpenter and alloy steel 304 and 31 2 for their extremeresistance to this type of caustic cor- 0 fro to 0.00069 for thesemetals with the protective film and o initial rate of corrosion beforefilm formation was a typical ferrous metal to corrosion resistance Theresults of these experiments lowing table.

Rate of corrosion of ferrous metals in IPY 10 rosion and a comparison ismade to show the relative effectiveness of the corrosion inhibitor ofthe vention in bringing flange iron,

alloy which is readily corroded comparable to that of the more resistantand more expensive a lloy steels.

are set forth in the fol 1 Tert.-octyl mercaptan. 2 Tert-butylmercaptan. Tert.-hexyl mercaptan (under N1 atm.).

All determinations were made in an atmosphere of larger (0 to 0.0178IPY) with the time for film formation air except run No. 14 wherein anatmosphere of nitrogen being 70 to 300 hours.

The samples all lost more weight immediately after immersion in the hotMercapsol soluwas used. During the last 378 hours of run No. 11 withflange iron and all of runs 12, 13, and 14, two flange iron tion thanafter passivation by the formation of a protective samples were tested,one insulated from the stainless film. The film formed was a blackscale, not steel disc turbine and designated flange iron #1, and thereadily attacked by strong acids and difficult to remove other notinsulated and designated flan lllium-G is the only alloy devoid of suchfilm. For these determinations, it vation and the rate of corrosionafter passivation are dependent on th ge iron #2, giving by rubbing withan emery cloth. a direct comparison in corrosion rates under otherwiseThis caused no apparent change in can be concluded that the time ofpassi 5 degree of agitation during the tests. Conditions of mildagitation (runs 2 and 5) gave more consistent results while rotation ofthe samples at a constant s feet per second through the Merc gave themost consistent data.

evidenced from the sample in the In run No. 12, from the 305 hour time 6to 428 hour time, the following sulfur compounds were found by analysisto be present: 0.6 weight volume per cent of sulfate and 0.1 weightvolume per cent of S203. In run No. 13, iron sulfide (FeS) was presentfrom 351 hours to 501 hours in 0.4 weight per cent concentration. thoughgiving accelerated rates A complete fresh charge of Mercapsol solutionwas not equivalent to the rates of corrosion experienced in certain pipefittings and other equipment made of flan iron in a Mercapsol unit.

added at 334 hours at run No. 11. No make-up inhibitor was added orneeded during the determinations.

Referring to Table II, runs 1 through 6, having as Consequently, in afurther study of the corrosion osion problem in which additionaldeterminations were made their objective the determination of the rateof corr 7 to ascertain the effect of ferric oxide alone and the additionof other corrosive sulfur compounds, the unusual inhibiting effect offerric oxide was discovered. The efficacy of this substance as acorrosion inhibitor is illustrated by runs 8 through 14, inclusive, inTable II. Herein it is seen that the corrosion rates are reduced toabout one-twelfth that of the flange iron in straight Mercapsolsolution. Although the concentration of ferric oxide was varied from0.04 to 0.6 weight per cent and the relative corrosivity of the causticincreased by addition of mercaptan, no conditions were found whichdisturbed the inhibiting action of the ferric oxide at optimum.concentrations of inhibitor. It is to be observed that with aconcentration of ferric oxide of around 0.18 weight per cent, themaximum reduction in corrosion rate amounting to about one-sixteenththat of the blank is obtained.

From these experiments, it is seen that small quantities of inhibitorsof the present invention may be used to accomplish the desired result.Since ferric oxide is soluble in alkali solutions with the formation ofalkali metal ferrites, the sodium salt being representative, Na2Fe2O4,it is apparent that the inhibiting action may be attributed to either orboth of these materials. No added results are obtained by usingquantities of ferric oxide in excess of the range of optimumconcentrations. The presence of small amounts of suspended particles offerric oxide in the alkali solution is not deleterious, except that theuse of such quantities of ferric oxides or alkali metal ferrites as toform a sludge which may foul the conduits or pumps in the system is tobe avoided. The concentration of the ferric oxide either as such or inthe form of th alkali metal ferrite may, therefore, be as low as 0.04weight per cent, and it is immaterial whether quantities are used inexcess of the solubility of the ferric 3 oxide in the alkali. Formaximum protection under most conditions it is contemplated that from0.04 to 0.6 weight per cent of ferric oxide, for example, may be used,although the preferred range is about 0.1 to 0.2 weight per cent toobtain the most efficient inhibition.

The optimum concentration of 0.18 weight per cent of inhibitor as usedin accordance with this invention serves to bring the rate of corrosionof flange iron down to that of the expensive stainless steels. Inaddition, the inhibitor of this invention successfully reduces the rateof corrosion of such alloys as that represented by 304 and 410. Theconclusions herein drawn are vertified by reference to the graphs,Figures 1, 2, 3, and 4.

Electron diffraction analyses were conducted to determine thecomposition of the scale that was formed on the metal surfaces. Theseanalyses showed that the scale was composed predominantly of ferrosicoxide (F6304) with strong possibilities of some ferrous oxide (FeO). Noevidence was found of ferric oxide or sodium ferrites in the scale. Thisserves to confirm in part the conclusion that the action of the ferricoxide and alkali metal ferrite inhibitors of the present invention isone of true inhibition or negative catalysis and not a mere blanketingof the metal from the influence of the alkali through some type of filmformation.

In the graphs, Figures 1, 2, 3, and 4, wherein weight loss in milligramsper square centimeter of sample surface is plotted against time andhours, the various curve numbers shown correspond to the run numbers setforth in Table II. For example, runs No. 7 and 8 of Table ll, which areconducted for the various test samples with no inhibitor present, arerepresented by curves 7 and 8 in each graph for the different alloysamples, and run No. 9, conducted with 0.60 weight per cent of ferricoxide, present, is represented by curve 9 in each graph for theindicated alloy, etc. And, likewise, run No. 14,

conducted with 0.18 weight per cent of ferric oxide and 1.0 weight percent of tertiary hexyl mercaptan in an atmosphere of nitrogen, isrepresented by curve 14. In Figure 1, it will be noted that curves 11,12, and 13 are shown in duplicate. The left-hand curve of each pairindicates the results of those determinations conducted after removal ofthe insulating washer from between the sample and the rotating discturbine. The curves graphically represent the data of Table II and pointup the unusual inhibiting effect of ferric oxide in transposing theresistive of flange iron into that approaching the more resistivestainless steel and nickel alloys.

An exemplary operation wherein caustic solutions con taining corrosivesulfur compounds are subjected to conditions conducive to the type ofcorrosion just illustrated by Table II is the regeneration of spentcaustic containing about 12 per cent by weight of free alkali and about0.25 weight per cent of rnercaptan sulfur. In such a regenerationsystem, the spent caustic is withdrawn from storage at a temperature ofto 100 F. at a rate of 20 to 30 barrels per hour and is heated to about220 to 230 F. by indirect heat exchange with steam for entry into anupper section of a regeneration tower operating at about 4 pounds persquare inch. Steam at 400 F. is fed to a reboiler operating inconnection with the tower to maintain temperatures at about 240 to 245F. therein. The approximate temperatures at the top and bottom of thetower are maintained at 212 and 250 F., respectively. Hot regenerated,concentrated caustic containing about 0.06 weight per cent of mercaptansulfur leaves the reboiler, passes in indirect heat exchange withincoming spent caustic, where it drops to a temperature of about 110 F.,and passes back to the hydrocarbon treating tower. In such operations,the addition of ferric oxide to the system in accordance with thisinvention will greatly reduce corrosion.

Regeneration by blowing the spent caustic with an oxygen-containing gas,wherein temperatures of F. and higher are encountered and hot causticmust be circulated by pumps and conduits, comprises another processwherein the present invention may be practiced to reduce corrosion.

The present invention may also be practiced by using separatecompositions comprising the inhibitor dissolved or suspended in asuitable vehicle, which compositions may be added to the corrosivealkali metal hydroxide solutions to be inhibited. Suitable vehicles forthis purpose may comprise a solvent or suspending medium, as water, oil,aqueous alkali, and portions of the corrosive alkali solution, as theMercapsol solution. Such soluticns of ferric oxide and/or alkali metalferrites in a vehicle may be prepared in concentrations designed toattain, when added to the corrosive alkali, sufficient inhibitor thereinto function in mitigating corrosion.

The rate of corrosion of ferrous metals by alkali solutions increasesalmost directly with increase in temperature and is most severe attemperatures of 250 F. or higher. However, the invention is not to belimited by any temperature range since caustic solutions exhibitcorrosivity at low temperatures, that is, 60 to F., depending on therate of flow of the caustic through the equipment. In ordinarycirculatory systems where caustic solutions are transported from tanksto treating towers and recycling is practiced, the rate of flow of thesolutions within the tanks heating coils, heat exchangers, pumps, andconduits will range from 0 feet per second to 20 to 25 feet per secondor higher. The volume rate of feed of caustic solutions through a givensystem may be as high as 50 to 100 barrels per hour depending on thesize of conduit used and the capacity of the system. The degree ofturbulence and impingement of caustic against the metal surfaces willvary throughout the system depending on the flow characteristics of theconduits. Since the inhibitors of the present invention are operativeunder static conditions and shown to be more effective where the degreeof turbulence and corrosion are higher, no upper limit need be set onthe rate of flow.

The invention is not to be limited by any theories expressed or impliedherein. It may be postulated, however, that several influences ortheoretical reactions may be operating singly or in concert to producethe results observed. One contributing cause of the corrosion may besmall amounts of oxygen present in the alkali solutions, as evidenced bythe reduction in corrosion found in test 14 (Table II) conducted in anatmosphere of nitrogen. Some nickel alloys seem to increase in theirrate of corrosion upon the exclusion of oxygen which would suggest thepresence of a protective film of nickel oxide as a reactant. Thecorrosion may be caused by a galvanic action which is upset by thepresence of the novel inhibitor'of the present invention due to a changein the nature of the scale formation. The influence of hydrogenembrittlement in the presence of sulfides in solution must not beoverlooked. The ferric oxide may act to screen the hydrogen ions fromthe metal or even prevent their formation.

It has been stated that the solution of ferric oxide in alkali producesalkali metal ferrites, and therefore alkali metal ferrites are includedas inhibitors herein. Certain alkali metal ferrites decompose in waterand are existent in stable form, mainly in strong alkali metal hydroxidesolutions, i. e., concentration of alkali of 40 per cent or more. Ifdecomposition of a ferrite solution takes place, there is formedhydrated ferric oxide or ferric hydroxide. Consequently, the inventionis not limited to the use of ferric oxide and/or alkali metal ferritesper se, and includes operable concentrations of hydrated ferric oxidewhich may form from such decomposition and also is directed to thosesolutions of alkali metal ferrites which are soluble in concentratedalkali Without decomposition under the conditions imposing in the systembeing treated.

What is claimed is:

1. In the method of regenerating spent aqueous alkali solutions whichhave been usedto extract sulfur compounds from hydrocarbon oils withoutsubstantial corrosion of ferrous equipment in contact with saidsolutions, said alkali solutions containing about to about 50% by weightof caustic alkali and containing phenolic organic material, but beingsubstantially free of alkali metal sulfides, the step comprising,incorporating about 0.04 to 0.6% by weight of an inhibiting material ofthe group consisting of ferric oxide and alkali metal ferrites in saidalkali solution throughout said regeneration as a corrosion inhibitor.

2. The method in accordance with claim 1 in which the ferrous metalsurface being protected is flange iron 0.42 manganese, and smallpercentages of sulfur and phosphorus.

3. In the method of regenerating spent aqueous alkali solutions used toextract sulfur compounds from hydrocarbon oils wherein said solutionsare subjected to heating temperatures ranging from about 212 to 250 F.in contact with ferrous metal surfaces under conditions conducive tocorrosion of said surfaces, the improvement comprising maintaining about0.04 to 0.6 percent by weight of an inhibiting material of the groupconsisting of ferric oxide and alkali metal ferrite in said alkalisolution throughout said regeneration to prevent corrosion of said metalsurfaces.

4. The method in accordance with claim 3 in which the inhibitingmaterial is ferric oxide in a concentration of about 0.18 percent byweight and said ferrous metal surface is flange iron.

5. The method in accordance with claim 3 in which the alkali metalferrite is sodium ferrite.

6. The method in accordance with claim 3 in which the alkali metalferrite is potassium ferrite.

7. The method in accordance with'claim 3 in which the alkali metalferrite is lithium ferrite.

8. The method in accordance with claim 3 in which the alkali solutioncontains about 19.3 percent by weight of sodium hydroxide, 10 percentcresols, 15 percent naphthenic acids, and 55.7 percent water.

References Cited in the file of this patent UNITED STATES PATENTS1,883,211 Wilson Oct. 18, 1932 2,132,585 Spittle Oct. 11, 1938 2,415,798Pye et a1 Feb. 11, 1947 2,556,387 Ayers June 12, 1951 OTHER REFERENCESment, published in Chem. and Metallurgical Chem.

Sept. 1944, page 125.

1. IN THE METHOD OF REGENERATING SPENT AQUEOUS ALKALI SOLUTIONS WHICHHAVE BEEN USED TO EXTRACT SULFUR COMPOUNDS FROM HYDROCARBON OILS WITHOUTSUBSTANTIAL CORROSION OF FERROUS EQUIPMENT IN CONTACT WITH SAIDSOLUTIONS, SAID ALKALI SOLUTIONS CONTAINING ABOUT 5% TO ABOUT 50% BYWEIGHT OF CAUSTIC ALKALI AND CONTAINING PHENOLIC ORGANIC MATERIAL, BUTBEING SUBSTANTIALLY FREE OF ALKALI METAL SULFIDES, THE STEP COMPRISING,INCORPORATING ABOUT 0.04 TO 0.6% BY WEIGHT OF AN INHIBITING MATERIAL OFTHE GROUP CONSISTING OF FERRIC OXIDE AND ALKALI METAL FERRITES IN SAIDALKALI SOLUTION THROUGHOUT SAID REGENERATION AS A CORROSION INHIBITOR.